In multicellular organisms, homeostasis is maintained by balancing the rate of cell proliferation against the rate of cell death. Cell proliferation is influenced by numerous growth factors and the expression of proto-oncogenes, which typically encourage progression through the cell cycle. In contrast, numerous events, including the expression of tumor suppresser genes, can lead to an arrest of cellular proliferation.
A particular type of cell death called apoptosis occurs in differentiated cells when an internal suicide program is activated. This program can be initiated by a variety of external signals as well as signals that are generated within the cell in response to, for example, genetic damage. For many years, the magnitude of apoptotic cell death was not appreciated because the dying cells are quickly eliminated by phagocytes, without an inflammatory response.
The mechanisms that mediate apoptosis have been intensively studied. These mechanisms involve the activation of endogenous proteases, loss of mitochondrial function, and structural changes such as disruption of the cytoskeleton, cell shrinkage, membrane blebbing, and nuclear condensation due to degradation of DNA.
The various signals that trigger apoptosis are thought to bring about these events by converging on a common cell death pathway, the core components of which are highly conserved from worms, such as C. elegans, to humans. In fact, invertebrate model systems have been invaluable tools in identifying and characterizing the genes that control apoptosis. Despite this conservation of certain core components, apoptotic signaling in mammals is much more complex than in invertebrates. For example, in mammals there are multiple homologues of the core components in the cell death signaling pathway.
Caspases, a class of proteins central to the apoptotic program, are responsible for the degradation of cellular proteins that leads to the morphological changes seen in cells undergoing apoptosis. Caspases are cysteine proteases having specificity for aspartate at the substrate cleavage site. Generally, caspases are classified as either initiator caspases or effector caspases, both of which are zymogens that are activated by proteolysis that generates an active species. An effector caspase is activated by an initiator caspase which cleaves the effector caspase. Initiator caspases are activated by an autoproteolytic mechanism that is often dependent upon oligomerization directed by association of the caspase with an adapter molecule.
Apoptotic signaling is dependent on protein-protein interactions. At least three different protein-protein interaction domains, the death domain, the death effector domain and the caspase recruitment domain (CARD), have been identified within proteins involved in apoptosis. A fourth protein-protein interaction domain, the death recruiting domain (DRD) was recently identified in murine FLASH (Imai et al. (1999) Nature 398:777-85).
Many caspases and proteins that interact with caspases possess a CARD domain. Hofmann et al. ((1997) TIBS 22:155) and others have postulated that certain apoptotic proteins bind to each other via their CARD domains and that different subtypes of CARD domains may confer binding specificity, regulating the activity of various caspases, for example.
Nuclear factor-kB (NF-kB) is a transcription factor that is expressed in many cell types and activates genes that have NF-kB sites in their promoters. Molecules that regulate NF-kB activation play a critical role in both apoptosis and inflammation. Inactive NF-kB resides in the cytoplasm complexed with the regulatory protein IkB. NF-kB binding to IkB causes NF-kB to remain in the cytoplasm. NF-kB activating stimuli activate specific IkB kinases that phosphorylate IkB leading to its degradation. Once liberated from IkB, NF-kB translocates to the nucleus and activates genes that have NF-kB sites in their promoters.
At least two dozen stimuli that activate NF-kB are known, including cytokines, protein kinase C activators, oxidants, viruses, and immune system stimuli. NK-kB is stimulated via signaling through the tumor necrosis factor family receptors (TNFRs) and the interleukin-1/Toll receptor. Tumor necrosis factor family members bind to their cognate receptors, including Fas (CD95/APO-1), TRAMP (DR3/WSL-1/AIR/LARD), CD37, CD30, CD40, TNFR1 and TNFR2, and regulate apoptosis, cell proliferation, and proinflammatory responses. For example, the proinflammatory cytokines TNF-a and IL-1 induce NF-kB activation by binding their cell-surface receptors and activating the NF-kB-inducing kinase, NIK. In the case of TNF-a, binding to TNF-R1 induces aggregation of its death domain and assembly of a signaling complex containing TRADD, TRAF2, and RIP. Binding of IL-1 to its receptor, IL-1R, induces aggregation of the receptor and assembly of a signaling complex which includes AcP, MyD88, IRAK1, IRAK2, and TRAF6. Both the TNF-R1 complex and the IL-1R complex trigger activation of NIK. Activated NIK phosphorylates the IkB kinases IkB-a and IkB-b which phosphorylate IkB, leading to its degradation and, as a consequence, the activation of NF-kB.
Fas, a cell surface receptor that is a member of the TNFR family, can induce apoptosis upon binding with its ligand, FasL (CD95L). Fas interacts with FADD (MORT) via death domains present in both proteins. When bound to Fas, FADD interacts with caspase-8 (FLICE/MACH/Mch5) through death effector domains present in both proteins. The complex of Fas, FADD and caspase-8 is referred to as the death-inducing signaling complex (DISC). Recently, FLASH, a protein having a DRD as well as a CED-4-like domain, has been identified as a component of DISC that is required for caspase-8 activation during Fas-mediated apoptosis (Imai et al. (1999) Nature 398:777-85). In the DISC, caspase-8 undergoes oligomerization-dependent autoproteolysis, leading to activation. Activated caspase-8 cleaves several effector caspases, including caspase-3, caspase-6, and caspase-7, by proteolytic cleavage. These effector caspases cleave various death substrates involved in the morphological changes and DNA fragmentation that is central to apoptosis.
Transient expression of FLASH activates caspase-8. However, a truncated form of FLASH lacking either its DRD or CED-4-like domain does not allow activation of caspase-8 or Fas-mediated apoptosis. Thus, it appears that FLASH is involved in both Fas- and TNF-induced apoptosis mediated by activated caspase-8 (Imal et al. (1999) Nature 398:777-85).
Bcl10 (mE10/CIPER/CLAP/c-CARMEN) is a CARD domain containing pro-apoptotic protein that induces NF-kB activation (Koseki et al. (1999) J. Biol. Chem. 274:9955-61; Yan et al. (1999) J. Biol. Chem. 274:10287-92; Thome et al. (1999) J. Biol Chem. 274:9962-68; Srinivasula et al. (1999) J. Biol. Chem. 274:17946-54)). Bcl10 activates NF-kB by acting upstream of NIK and IkB kinase (Srinivasula et al., supra). Significantly, Bcl10 is involved in t(1;14)(p22;q23) of MALT B cell lymphoma (Willis et al. (1999) Cell 96:35-45; Zhang et al. (1999) Nat. Genet. 22:63-8). Bcl10 expressed in MALT lymphoma exhibits a frameshift mutation that causes truncation of Bcl10 distal to its CARD domain. The truncated form of Bcl10 activates NF-kB, but does not induce apoptosis (Willis et al. (1999) Cell 96:35-45). Expression of NF-kB is associated with suppression of apoptosis and increased cell survival in certain systems. Thus, mutant Bcl10 may promote continued cell proliferation by two different mechanisms. Bcl10 mutations similar to that observed in MALT lymphoma occur in many other tumor types, suggesting that Bcl10 may be commonly involved in malignancy.
Murine FLASH is a protein involved in Fas-mediated activation of caspase-8 during apoptosis (Imani et al. (1999) Nature 398:777-85). Transient expression of murine FLASH activates caspase-8. It appears that the DRD domain (amino acids 1584-1751) and the CED-4-like domain (amino acids 939-1191) of murine FLASH are required for activation of caspase-8.
NF-kB and the NF-kB pathway have been implicated in mediating chronic inflammation in inflammatory diseases such as asthma, ulcerative colitis, rheumatoid arthritis (Barnes & Epstein (1997) New England Journal of Medicine 336:1066) and inhibiting NF-kB or NF-kB pathways may be an effective way of treating these diseases. Binding sites for the transcription factor NF-kB are present in the promoter regions of the genes of many of the proinflammatory cytokines, chemokines, enzymes, immune receptors, and adhesion molecules important in inducing acute inflammatory responses associated with critical illnesses. Because increased activation of NF-kB can lead to enhanced expression of proinflammatory mediators, NF-kB activation may be an important event in the development of, for example, multiple organ dysfunction associated with infection, blood loss, and ischemia-reperfusion injury (Abraham (2000) Crit Care Med 28(4 Suppl):N100-4).
NF-kB and the NF-kB pathway have also been implicated in atherosclerosis (Navab et al. (1995) American Journal of Cardiology 76:18C), especially in mediating fatty streak formation, and inhibiting NF-kB or NF-kB pathways may be an effective therapy for atherosclerosis. Among the genes activated by NF-kB are cIAP-1, cIAP-2, TRAF1, and TRAF2, all of which have been shown to protect cells from TNF-A induced cell death (Wang et al. (1998) Science 281:1680-83).